Semiconductor devices are becoming smaller and more dense with the evolution of new technology. However, increases in circuit density produce a corresponding emphasis on overall chip packaging strategies in order to remain competitive. Chip and substrate manufacturers are therefore constantly being challenged to improve the quality of their products by identifying and eliminating problems, reducing package size and weight, decreasing package costs, providing improved thermal efficiencies and better and more advanced chips. Whereas significant improvements are being made to eliminate systematic problems by reducing process variability, process improvements alone are not sufficient to eliminate all the problems which affect both performance and reliability.
One way to allow high degrees of integration is to provide a highly efficient internal cooling design. A preferred way of cooling high performance SCMs (single chip modules) and MCMs (multi-chip modules) is using thermal paste. Thermal paste is often used as a high thermal conductivity interface material to fill the gaps between the back-side of chips, such as, flip chips, and the inside surfaces of the caps. Producing modules that use thermal paste has multiple challenges, for example, the paste can adhere to the module components, such as, the back surface of the chip and/or the inside of the cap, or the module components should be chemically compatible with the thermal paste. Therefore, the package must be designed such that the thermal paste filled chip-to-cap gap has sufficient thickness that it will form a reliable structure, but not too thick, which could allow the paste to sag out of the gap over time.
U.S. Pat. No. 3,704,166 (Cuomo et al.) the disclosure of which is incorporated herein by reference, discloses a method for improving adhesion between a conductive metal layer (Tungsten or Molybdenum) and a substrate of insulating material (silicon dioxide) at deposition temperatures below 500.degree. C. Basically, the process involves cation substitution in the surface layers of the substrate lattice by low temperature solid state diffusion or ion implantation without modifying the bulk dielectric properties of the substrate. Metal is deposited on the modified substrate surface and the entire structure is heated below 500.degree. C. to achieve adhesion between the deposited metal and the dielectric substrate.
U.S. Pat. No. 4,634,600 (Shimizu et al.) basically discloses a method for forming a thin film on the surface of a substrate to provide a hard surface layer, such as, by forming and coating carbide, nitride, oxide or boride on a substrate (tool bit) in order to increase the abrasion and corrosion resistance. The hard surface layer is obtained by simultaneously vaporizing/ionizing metal and non-metal (Ti and N, for example) ions and accelerating them towards, and implanting them on, the substrate.
U.S. Pat. No. 4,849,247 (Scanlon et al.) discloses a method of applying high energy bondable ion to the surface of a substrate to create a bondable surface for bonding of similar or dissimilar materials.
U.S. Pat. No. 4,886,681 (Clabes, et al.) the disclosure of which is incorporated herein by reference, discloses a technique for improving metal-organic substrate adhesion and for reducing stress between the metal film and the substrate. Low energy reactive ions, electrons or photons are used to alter the surface chemistry of the substrate to a shallow depth between about 10 angstrom and a few hundred angstroms in order to enhance adhesion.
U.S. Pat. No. 4,917,953 (Hioki et al.) discloses a method of forming a solid lubricating film on a ceramic material.
U.S. Pat. No. 5,045,345 (Singer) discloses coating of bulk engineering material with oxidation, corrosion or wear resistant inorganic materials.
U.S. Pat. No. 5,185,184 (Koran et al.) discloses a means of promoting adhesion between a substrate and an activatable adhesive. An adhesion promoter layer that activates the adhesive and promotes bonding is embedded in the substrate surface by adding the activator to the sandblasting material and sandblasting the substrate surface.
U.S. Pat. No. 5,023,695 (Umezawa et al.) discloses a flat plate cooling (FPC). In this structure, a flat cooling plate is just above the array of chips. Thermal paste is used to fill the gaps between the chips and the flat plate.
U.S. Pat. No. 5,098,609 (Iruvanti et al.) the disclosure of which is incorporated herein by reference, discloses stable high solids, high thermal conductivity pastes. The pastes include a thermally conducting solid filler, a nonaqueous liquid carrier and a stabilizing dispersant. The resulting pastes are highly concentrated, of low viscosity, electrically resistive, highly thermally conducting and stable.
U.S. Pat. No. 5,591,789 (Iruvanti et al.) the disclosure of which is incorporated herein by reference, discloses a polyester dispersant for use in high thermal conductivity pastes.
U.S. Pat. Nos. 5,604,978 and 5,623,394, (Sherif et al.) the disclosure of which is incorporated herein by reference, disclose a method and apparatus for the customized cooling of chips on an MCM with a range of cooling requirements. It uses flat plate cooling, and uses pastes of different thermal conductivities on chips to customize the cooling of the chips. The paste is either between the chips and a flat cooling hat, or between the chip and a blind hole in the hat. Surplus paste may also fill some or all of the rest of the inside of the module.
U.S. Pat. No. 5,757,620 (Edwards et al.) the disclosure of which is incorporated herein by reference, discloses a method and apparatus for cooling of chips using blind holes with customized depth. It uses flat plate cooling, and varies the depth of the thermal paste filled gap to customize the cooling to each of the chips on a module, such as, a MCM (Multi-Chip Module).
U.S. Pat. No. 5,825,087 (Iruvanti et al.), the disclosure of which is incorporated herein by reference, discloses a hermetically sealed module where the internal surface of the module has a roughened surface by grit blasting or machined to have parallel and/or crossing grooves. The paste penetrates the roughened surface and inhibits the flow of the paste out of the gap.
A variety of surface modification techniques have been disclosed that enhance the adhesion between various materials. A variety of thermal paste cooling applications have been disclosed. Thermal paste offers a combination of low cost hardware, high thermal performance, and module reworkability. What is needed are new methods of improving the reliability of cooling designs using thermal paste or thermal paste like viscous thermal interface materials.